Original subtitles

[Music]

hello interwebs welcome to let's fix

computers i've got a macbook air here

which does not power on we have

no green light on the charger no orange

light no light whatsoever so

that means that we are missing some

primary main super important power rails

so let's take the bottom cover off and

have a look so i'll buzz out the uh 10

screws from the bottom

there's an odd coating

over this thing

that looks like um a very fine vinyl

wrap or something like that to cover up

a load of scratches or something

not sure why but if you're wondering why

there's loads of weird-looking markings

on it yeah that's what that is no idea

why that's there

oh that's interesting it's spray paint

and i tell you how i know that because

it's gone all up the inside

how

bizarre uh fine

we're missing a screw from there as well

it's been disassembled before because

this screw is scratched up so

okay

uh

let's see what do we have i'm just going

to take off the

bridge and just inspect that

right i've unplugged the battery and

plugged in the charger

let's check for some basic voltages

i'm guessing we're not going to have

pp3v42

because

that powers the light on the charger

um so we kind of need that

that'll do right

[Music]

is pp3v42 on this side i don't think it

is let's check for it over here

19 volts in

and there is pp3 v42

at the source so that is there okay

what about

what about pps g3 hot we got that

ah okay right pp bus g3 hard this tells

us a bit more

that is at

that is at 8.2 volts

uh which is wrong that should be 8.6

what this tells me is that the smc is

not running

uh because

pp bus g3 hot will run at 1.8

or 8.18

and then when the smc switch is on it

bumps it up to 8.6

so from this we can determine that the

smc is not working and that is also why

we don't have a light on the charger

because the smc controls that light

so what we'll do now is i'm going to

take the logic board out of this thing

and i'm going to see what we find on the

other side of the board

um i'm given that we've got just spray

paint on this thing

i'm concerned about what's on the other

side of the board let's take it apart

what

all right so again we've got this line

of silver along the back it hasn't

strayed too far onto the logic board

i don't think that's done any damage

i don't think this is the cause of our

problem

at the very least

i'm going to plug this back in just to

check if there's been any change by

removing the motherboard from the laptop

there has not

i might grab a different dc in board as

well just to make sure i'm not getting

punked by a bad dcn board

because if this guy's not working

properly

um then that might be causing our issue

and that might cause the smc not to turn

on which means you have nothing

so i'm going to swap out the dc board

just in case

all right different dc board

bam

yeah same thing okay

fine

why doesn't rsmc turn on

so

let's pull up a schematic for this

and we'll just check some obvious things

so this is a

8200 165

and if i zoom down to the corner here

there's a section down here and this is

smc reset

and what this chip does is

it tells this it tells the smc if it

should turn on or not basically

so it sends the on off signal the reset

signal that kind of thing so what i want

to know is is the smc being told to

switch on

uh or is it in a reset state

is there some other reason why it's not

turning on or is it just busted

so i'll put my multimeter into voltage

mode again

and let's just see what i want to probe

so firstly

smc reset l is the first one i want to

check

so smc reset if a device is in reset it

means it is resetting

so a reset signal is a momentary signal

it's like the reset button on your pc

when you press it you momentarily

contact that switch and then release it

again while the switch is in while the

button is in the computer is in a reset

state and then when you release it it

comes out of a recessed state and begins

to boot up again

so

in this case smc reset underscore l the

l means low so it means when this signal

is low a la 0 we are in a reset state as

if we're holding down the reset button

so we want to see

a signal here so i'll put my black probe

on a screw hole for ground and i want to

check pin five

um

it's going gonna be a little bit awkward

to get in there let's see if we can

probe that guy

and there's

1.2 volts there

not sure if that's correct or not i was

expecting um

i was expecting

a 3.3 volts

what is powering this chip

this chip is powered by pp3v42

let's make sure that it's getting

3.42 volts

that might be correct you know might

just be 1.2 is the high state

i was just expecting 3.42 volts because

that's normally a

3.42 or 3.3

is normally the true signal state for a

macbook

i believe anyway

uh okay that's fine what about smc on

offl what state is that in so that's pin

seven

the guy goes over to this big old pad

here

that is high

okay where does smc reset come from

it's got a pin over a j tag over here

which got spray painted

i can't believe that that's our issue

i'm gonna look up the schematic for this

to find out how smc reset is created

because i'm not sure

okay yeah that's confirmed um the u5110

does create smc reset l

and also this line it should be i was

correct it should be at 3.40 volts

because here we have a line pulling it

up this is called a pull-up resistor so

it's a 100k resistor and what this does

is it's got pp3v42

coming down onto this via this resistor

so

because this resistor connection is here

in a normal condition smc reset l should

be at

3.42 volts

and then when reese

when we want to reset the smc

this chip will pull this to ground

now you might say well if that grounds

it doesn't that create a short circuit

it doesn't because of the resistor

because there's a 100k resistor there

when you short this to ground

uh it cannot go short circuits there's

100k of resistance to the power line so

this goes to ground but the pull up line

does not which is why it's not a short

circuit

so that's why when

chips are sending signals to each other

they can just short something to ground

because the natural state is going

through a pull-up resistor

so this confirms that our reset should

be at 3.42 volts

so we need to find out why it's not

because i think that's our problem

um 1.2 volts it's not zero but that's

probably low enough to be interpreted as

a reset state so we need to find out why

so what else is on that same smc

reset line

so we had the jtag connector

and we've got

a test point all the way up here

is that even visible

no it's not okay what's on the other

side of the board

then on the other side of the board

there's a resistor over here going to

what's that

i can't remember what the u7100 is

it looks like um charger management

and then we've got the actual smc itself

which is the new 5000

so there's not a huge amount on here

let's start with the simple one let's

take u5110 off the board and see if smc

reset goes high

now we won't be able to turn on with

this thing off the board because

there'll be nothing to tell the smc to

turn on but what we need to do is just

see if that pull-up resistor then pulls

up to 3.42 volts

so let's see if i'm lucky and it's that

simple

because i was kind of hoping that there

wouldn't be a lot more on that line but

there's a couple of things and yeah

right chip removed

so

i'm just going to give the board a

second to cool down

we have a green light on the charger

and was that fan spin

it shouldn't start

yeah that's what i thought good okay

i was gonna say we should be quite

unable to turn on at the moment because

the chip that tells the smc to turn on

is not there

however it looks like we have a culprit

there we go so smc reset l is now up at

three point well 3.38 so it's supposed

to be 3.42 but probing and stuff like

that that line should be 3.42 but

because i'm trying to poke it with tiny

little pro points and there's losses in

the wires and all the rest of it you'll

never see exactly the right voltage but

that's clearly supposed to be 3.42

um fine so u-5110 is dead um and that

seems to be

partially shorted to ground it's not a

full short to ground

however it was certainly dying enough

that it was half pulling down that that

line

so theoretically if i can find another

one

we can just stick another one on the

board and we're laughing so i'm going to

check my breaker boards and see what

i've got

this board's a bit grubby around that

area however i had no reason to believe

that this one doesn't work we'll soon

find out

no that's really not coming clean fine

whatever

hot air and flux will boil it all clean

purity is coming

[Applause]

ah

that came off discern concerningly

easily

and for orientation the dot is in the

bottom right of the chip

and that is pin one

so i'll just put that on the board there

for a sec

and if i check the schematic we'll find

pin one is indeed in the bottom right

and we'll just touch up these solder

joins before we put this back on just so

we get a decent connection

part me is very tempted just to put the

same chip back on the board just for

testing

i might do that because it's not

uncommon to have just a chip that's just

developed a cracked solder joint or a

dry joint

and

it's interesting to test for that

because sometimes you don't actually

have to replace the part and if you just

happen to not have a spare

then that's suddenly going to be very

interesting

so i'll actually put the old one back on

first just for the sake of testing

all right that goes back on let's just

see what happens now

if you've got replacements then i would

recommend using them anyway i think

just in case you've got an intermittent

part but

i'm interested in doing this for testing

we have a green light and we have fan

spin

and the fan continues to spin because

we've actually switched on now

there you go folks didn't even need the

spare

good stuff

all right well i'm going to put the old

one back on my donor board then just so

it's still there

and then we'll put this back together

again

so this one turned out to be easier than

it looked however cracking example of um

of smc reset l

in action

or rather not in action

and also a great example that

you might not need to replace the bit

that you've taken off

as i say that's not necessarily the

recommended action

but i know certainly for people who are

starting out in board repair

you probably don't have a load of

breaker boards lying around

and

when you don't have heck i need some

flux on here i was trying to be lazy

there we go

when you don't have um all the spare

bits lying around suddenly very simple

problems can be very difficult to deal

with because

you've got to spend three weeks waiting

for a bit to show up from china every

time you make a move

which is a hecking pain in the ass

ask me how i know

all right i don't need this to be on

properly i just need it to be on

that should do

as long as it doesn't long as it doesn't

fall off

there we go good

so i wonder if that was the um

uh the spray paint that's eaten away at

that

or whether that's just a coincidence

either way don't spray paint your

laptops

in this manner

if you're going to spray paint something

actually take it apart and strip it down

first

and this doesn't happen

what a bizarro issue

there we go that looks fine to me

let's put it back in the laptop

reassembly is the worst bit because this

is the point where you're just like i've

won i've done it can i go and do

something else now

actually i'll rephrase that the worst

bit isn't risk the worst bit is

reassembly

after you've given up

because

you have to put the laptop back together

again because you can't just give it

back to the customer in a pile of pieces

but at the same time it feels like such

a waste of time to reassemble a dead

laptop

i try to reassemble using the minimum

amount of screws

just in case i have to take it apart

again but by the time you've actually

gone to the trouble of

connecting everything up and plugging it

all in for a fair test

you may as well just put in the last

couple of screws

especially considering this is missing a

couple anyway

so we're going

no guts no glory now

let's have a battery as well because why

the hell not

speaker right

remember folks don't look at what's in

the laptop look at what is left on your

bench and the only thing as i have left

on my bench are the case screws and the

spare dc inboard that i was testing with

all right

because if there's nothing left on your

bench then you must have put it all in

the laptop right

bam green light

charge

and chime

huh

and get power cycled

there it goes

right

lovely

we have a working laptop everyone

there we go

so a bad connection under

u5110

was causing the smc to be stuck in reset

mode which was preventing the laptop

from turning on

and reflowing the reflowing the solder

joins on that chip seems to resolve the

issue

i'm going to leave this to one side just

to make sure it doesn't cut out and then

i'll give it some reboots and stuff like

that and assuming that there's no

problems there it goes back to the

customer thank you very much for

watching everyone as always my support

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thank you and see you next time bye

you